1. Field of the Invention
The present invention relates to a fuel cell system for supplying a current to a load by generating electricity using a chemical reaction between hydrogen and oxygen, and a method for controlling air-flow rate therein.
2. Description of the Related Art
Fuel cells, which have been widely used in various fields including vehicles (e.g., automobiles), domestic applications and industrial applications, generate electricity using a chemical reaction between hydrogen and oxygen. Therefore, when load of the fuel cell becomes higher, an amount of hydrogen discharged by purging, draining or the like during an operation of the fuel cell becomes larger, so does a flow rate of air containing oxygen to be reacted with hydrogen.
FIG. 9A is a graph showing a relationship between load and air-flow rate in a fuel cell, in which the abscissa shows the load of the fuel cell and the ordinate shows the flow rate of air supplied to the fuel cell. FIG. 9B is a graph showing a relationship between load of a fuel cell and amount of hydrogen discharged from the fuel cell, in which the abscissa shows load of the fuel cell and the ordinate shows amount of hydrogen discharged.
Conventionally, when the air-flow rate is small under low load (see FIG. 9A), the amount of air for dilution is controlled in accordance with a set range h, which is a hydrogen discharge amount per purge operation under high load (see FIG. 9B), in order to suppress a raise in the hydrogen concentration.
Alternatively, the volume of the diluter configured to store discharged hydrogen is made larger to some extent so that the diluter can temporally store a larger amount of the discharged hydrogen in order to prevent the hydrogen concentration from increasing when the load is reduced and to suppress the hydrogen concentration to a specific low concentration.
For example, since the diluter into which hydrogen containing impurities is to be discharged is exclusively filled with air before purging of such hydrogen, the concentration of the hydrogen to be exhausted can be reduced by making the volume of the diluter (space for storing hydrogen) large.
FIG. 10 is a schematic cross section showing a configuration of a conventional diluter 101 and vicinities thereof.
As shown in FIG. 10, into a hollow space of a diluter 101, hydrogen discharged from the fuel cell (hereinafter, frequently and simply referred to as “discharged hydrogen”) flows from a hydrogen discharge piping 102 through an opened purge valve or drain valve, as indicated with an arrow c. By air flowing through an air discharge piping 103 as indicated with an arrow a which is to be exhausted out of the vehicle (hereinafter, frequently and simply referred to as “discharged air”), the discharged hydrogen in the diluter 101 is sucked from a hydrogen outlet 103d formed in the air discharge piping 103 as indicated with an arrow A, and then exhausted out of the vehicle together with the discharged air, as indicated with an arrow b. The expelling of the discharged hydrogen in the diluter 101 from the hydrogen outlet 103d to the air discharge piping 103 is facilitated by air for diluting and pushing out, which is introduced from a bypass piping 104 as indicated with an arrow d (see JP2006-155927A, especially paragraphs 0028-0033, 0042, 0044, and FIGS. 1 and 2).
As described above, by making the volume of the diluter 101 larger, the concentration of the total discharged hydrogen in the diluter 101 that has flown thereinto becomes lower, and thus the amount per unit time of the discharged hydrogen can be lowered (equalized) at the hydrogen outlet 103d where the discharged hydrogen is mixed with the discharged air (see FIG. 10). With this configuration, a raise in the discharged hydrogen concentration in the air discharge piping 103 can be suppressed, even when the flow rate of the discharged air in the air discharge piping 103 decreases.
On the other hand, in JP2005-11654A, there is disclosed a fuel cell system with a diluter for diluting hydrogen to be exhausted, in which a dilution gas supply passage for introducing discharged air from a fuel cell to the diluter is branched from a discharged oxygen-containing gas exhausting passage on which a pressure-regulating valve is provided, and an opening degree of the regulating valve is reduced to increase the amount of the discharged oxygen-containing gas to be introduced to the dilution gas supply passage when the amount of the discharged oxygen-containing gas is low (see paragraphs 0007 and 0010, FIGS. 1 and 2 and the like).
Meanwhile, in JP2006-155927A, the amount of the discharged hydrogen is reduced by reducing an opening period of the purge valve (i.e., reducing a purge amount per purge operation), while increase and decrease of the air-flow rate caused by change in the load of the fuel cell are taken into consideration. In addition, setting is made in such a manner that a raise in the concentration of the discharged hydrogen is suppressed when the amount of the discharged air is small. However in this case, a purge effect per purge operation becomes small, and as a result, an amount of hydrogen efficiently used for electricity generation becomes low, leading to a problem of increase in the hydrogen discharge amount.
Further, in the case where an increase in the hydrogen concentration is suppressed by making the diluter 101 for storing the discharged hydrogen larger, the weight of the diluter 101 becomes larger, the fuel economy becomes lower, and the material cost for the diluter 101 becomes higher, leading to a problem of the higher cost.
On the other hand, in JP2005-11654A, when the amount of air discharged from the fuel cell is small, the air amount more than the amount of the discharged air cannot be introduced to the diluter, no matter how the opening degree of the regulating valve is controlled. This leads to a problem that, for example, in the case where hydrogen is discharged under high load and then the load becomes low in a short period of time, difficulty arises in securing an amount of air required for diluting a large amount of hydrogen discharged under high load
As described above, when the load of the fuel cell rapidly decreases from a high level to a low level, the discharged air amount decreases in a shorter period of time, relative to a time period from immediately after purging hydrogen under high load to discharging hydrogen out of the system under the decreased load. As a result, there arises a problem that the hydrogen concentration may increase above an allowable range, due to this time lag.
Therefore, it would be desirable to provide a fuel cell system which is capable of appropriately controlling the air-flow rate in accordance with the amount of the discharged hydrogen, and controlling the hydrogen concentration in gas discharged from the system to be retained in an allowable range. It is also desirable to provide a method for controlling the air-flow rate as such.